The present invention relates to the manufacturing of semiconductor devices, and more particularly, to reflow processes that generate flux residue.
An increasingly important aspect of manufacturing an integrated circuit chip, also referred to as an integrated circuit die or semiconductor die, is the mounting of the die to a substrate. Often times, the goal of this process is to provide the chip with as many input/output (xe2x80x9cI/Oxe2x80x9d) terminals as possible. Because of its capability to provide high I/O density, flip-chip bonding is one of the various surface mounting techniques being used in the semiconductor industry. In addition, flip-chips provide small profiles and good electrical performance.
In the flip-chip bonding process, the die is mounted directly to the substrate. A representation of a flip-chip 10 is illustrated in FIG. 1. Generally, the flip-chip process entails disposing a plurality of solder bumps 12 on the upper surface 14 of the die 16, flipping the die 16 and mating these solder bumps 12 with corresponding bonding pads 18 located on the substrate 20. The combination of the die 16 and substrate 20 is then heated so as to reflow the solder bumps 12. Once reflowed, each bump 12 forms a bond between the die 16 and the substrate 20, with the bond functioning as both an electrical and a physical connection.
The metallized surfaces to be bonded are typically heavily contaminated with metal oxides, carbon compounds, and other materials due to extended exposure in the manufacturing environment and therefore require cleaning prior to bonding as a metallized surface contaminated by these materials is difficult to be wetted by solder. However, once this surface contamination is removed, the solder can wet the metallized surface and form a metallurgically sound solder joint, which will both hold the various electronic components in place and pass electrical signals.
These contaminants are typically removed from the metallized surfaces by the application of fluxes. A typical flux consists of active agents dissolved or dispensed in a liquid carrier, such as a flux paste. The carrier for flux is typically alcohol-based, with varying concentrations of acids or salts as activators. The function of the activators is to reduce base metal oxides. The flux has a variety of purposes, which include removing oxides from the metallization; removing oxides on the molten solder to reduce the surface tension and enhance flow; inhibiting subsequent oxidation of the clean metal surfaces during soldering; and assisting in the transfer of heat to the joint during soldering.
Depending upon the type of flux paste, a flux residue remains after reflow welding during which the solder joint is formed. The residue comprises a carrier, such as rosin or resin that is not evaporated, acid or salt deposits, and the removed oxides. If not removed, this residue can be detrimental to the long-term reliability of an electronic package. The resin can also absorb water and become an ionic conductor, which could result in electrical shorting and corrosion. Additionally, the residual activator can, over a period of time, corrode the soldered components and cause electrical opens.
If fluxes that leave corrosive and/or hygroscopic residues are used, post-soldering cleaning using chlorinated fluorocarbons (CFCs), organic solvents, semi-aqueous solutions, or water is required. For this type of process, in addition to volatile organic compound emissions from the soldering process, the cleaning process results in emission of CFCs and waste water. These emissions detrimentally add to environmental pollution and production costs.
A type of flux that advantageously does not require cleaning is commonly referred to as a no-clean flux. This type of flux is designed to leave little or no residue, thereby negating the need to clean the semiconductor device after reflow welding. No-clean fluxes were originally intended for such operations as board mounting in cell phones, motherboards, or PCs for which the main concern of the manufacturer was leakage. Also, the flux was designed so that even if a residue remained, the residue would not be detrimental to the long-term reliability of the semiconductor device. However, this residue can cause a problem during a process known as underfilling.
Underfilling has been used to solve a problem in flip-chip mounting caused by a mismatch commonly found between the coefficient of thermal expansion of the semiconductor die and that of the substrate. Because of thermal gradients experienced by the semiconductor device during normal operation, the solder bumps which couple the die to the substrate experience significant stresses. These stresses can cause thermal fatigue and connection failures. Underfilling has been commonly used to overcome the thermal mismatch between the die and the substrate. This process involves inserting an encapsulation material, such as epoxy resin or other material, into the space between the semiconductor die and substrate after the die has been soldered to the substrate. In addition to being inserted into the space, surface tension produces a capillary action between the die and the substrate which pulls the epoxy into the space. This encapsulation material surrounds the solder bumps and mechanically couples the die and the substrate, thereby decreasing the stress in the solder joints to improve the lifetime of the semiconductor device.
The use of no-clean fluxes before underfilling, however, can result in voids in the encapsulation material. In accordance with prior art methods, as illustrated in FIG. 2, the distance between the semiconductor die 16 and the substrate 20 is very small, for example 3 mils or smaller, and any residue 22 left by the no-clean flux after reflow can act as a physical barrier to the encapsulation material 24 to the space 26 between the semiconductor die 16 and substrate 20. Also, flip-chips are characterized by relatively small pitches between solder joints, which further reduce the area into which the encapsulation material 24 can flow. Therefore, although this residue 22 may otherwise be benign, the blocking caused by the residue 22 can cause voids 28 in the encapsulation material. Voids 28 are a problem because any void 28 in the encapsulation material 24 adjacent a solder bump 12 reduces the stress-relieving properties of the encapsulation material 24. A similar type of problem also exists during underfilling between decoupling capacitors and a substrate. Accordingly, a need exists for an improved method of bonding using no-clean fluxes that reduces or eliminates the residues produced after the reflow process.
This and other needs are met by embodiments of the present invention which provide a method of attaching a component to a substrate using solder to form a semiconductor device, comprising applying flux to the semiconductor device; heating the solder and the flux in a furnace to bond the semiconductor die to the substrate; and controlling moisture content of an atmosphere surrounding the flux.
By controlling the moisture content of the atmosphere surrounding the flux, the present invention reduces the amount of flux residue formed during the reflow process, thereby avoiding the creation of voids in the encapsulation material of the semiconductor device and reducing ionic contamination that can compromise the reliability of the semiconductor device by leakage.
A further aspect of the present invention includes controlling the moisture during the heating step. Controlling the moisture can include the steps of measuring the moisture content in the furnace and providing a signal when the moisture content exceeds a threshold amount. In certain embodiments, the moisture content is maintained below 50 ppm. However, in other embodiments, the moisture content is preferably below 20 ppm.
Another aspect of the present invention is that the flux used during certain embodiments of this process is a no-clean flux. Also, the components being attached to the substrate can be flip-chip semiconductor dies or chip capacitors, which typically require an underfilling operation after reflow. The temperature of the furnace during reflow is from about 220xc2x0 C. to about 380xc2x0 C. in certain embodiments.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the resent invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.